This invention relates to a method and apparatus for centrifugally separating particulate material from a liquid phase and, more particularly, to a method and apparatus for the batch separation of blood components.
It is known to use centrifugal techniques for the separation and/or fractionation of particulate materials suspended in a liquid according to particle density, size, shape, etc. Unfortunately, during centrifuge deceleration, and subsequent removal of the separated constituents, there is a tendency for remixing of the separated components. This is particularly true in the case of blood.
The separation of blood into cellular components and plasma, in general, and preparing platelet rich plasma, in particular, has become of great interest to the medical community. The increased use of chemotherapy and other techniques requires platelet concentrate transfusions. Unfortunately, present blood bags and many-batch type blood separation techniques do not facilitate good platelet separation. There is always some incipient remnants or traces of red blood cells and white blood cells. Antigens on certain these blood contaminants give rise to alloimmunization of the recipients of such transfusions, thereby reducing the efficiency of subsequent transfusions. It therefore becomes necessary in many cases to select and type the donors--it being no longer possible to use random donors. This greatly increases the cost. Some of the problems incipiently related to alloimmunization are described in an article entitled, "Correction of Poor Platelet Transfusion Responses with Leukocyte-poor HL-A-matched Platelet Concentrates" by R. H. Herzig, et al., Blood, Vol. 46, No. 5 (Nov.), 1975.
A frequently-use blood component separation procedure involves the preparation, in two centrifugation steps from a single-donor unit of whole blood, of a packed red-cell fraction which also contains most of the white blood cells, a concentrate of platelets suspended in plasma, and a platelet-poor plasma fraction. During the first centrifugation, red blood cells sediment to the bottom of the centrifuge bucket (i.e., pack at the bottom of the blood bag which is oriented horizontally during centrifugation in a swinging-bucket rotor) and a platelet-rich plasma layer extending from the top of the bag to the red-cell interface region is formed. White blood cells are concentrated in the plasma layer immediately above the packed red-cell mass (the so-called buffy-coat region) as well as in the upper portion of the packed red-cell region. After rotor deceleration, the platelet-rich plasma layer is expressed into a satellite bag leaving the packed red cells and buffy-coat layer in the original draw bag. The platelet-rich plasma is then centrifuged to sediment the platelets, after which most of the platelet-poor plasma is expressed into a second satellite bag, leaving a platelet concentrate in the first satellite bag.
One of the factors contributing to contamination (unwanted levels of white and red blood cells in platelet-rich plasma) of the platelet concentrate, giving rise to alloimmunization, is the formation of folds in the upper part of the blood bag during centrifugation. These folds permit the red and white blood cells to become entrapped in the folds hence expressed with the platelet containing plasma causing some of the undesired contamination noted above. This tendency to fold in the top portion of the bag can be aggravated by the fact that satellite packs, tubing and balancing pads are usually placed within the bucket with the blood bag. Further, the technician in removing the bag from the swinging bucket of the centrifuge, as well as in subsequently handling the bag, can cause some disturbance and remixing of the bag's contents.
A second major factor contributing to unwanted contamination of the platelet concentrate is the phenomena which occurs during the final stages of deceleration of the centrifuge rotor. The deceleration of a unit of fluid on the extreme outboard side of the swinging bucket as it reassumes a vertical orientation will be greater than that of a unit on the extreme inboard side. This results in a fluid rotation about the bucket center unit. The rotating or swirling fluid tends to cause some remixing of the components, which were separated during centrifugation, before they can be expressed from the bag into the satellite bags. Efforts in the past to reduce this swirling have been directed to decreasing bucket diameter, using oval buckets, and the like. Long, thin buckets greatly enlarge the size of the centrifuges and hence generally are not a practical solution. Further, the thin, long tubes increase centrifugation time. Swirling can be reduced by increasing centrifugation deceleration time, but this severely reduces throughput and hence greatly increases processing costs.